The present invention relates to a magnetic bearing device used in a turbo-molecular pump, a machine tool, or the like.
Magnetic bearing devices are often used as bearings in turbo-molecular pumps, machine tools, or the like, which require high-speed rotation with low vibration. In the case of a rotating body, a five-axis control type magnetic bearing is generally used. In a five-axis control type, four-axis control is performed in the radial direction and one-axis control is performed in the axial direction. Each axis is provided with a displacement sensor for detecting displacement between the magnet and the rotating body, and an inductance type sensor is often used for the displacement sensor. Usually, an opposed type is used on the radial axis and a non-opposed type is used on the axial axis.
A carrier wave is applied to the displacement sensor, and the carrier wave is amplitude modulated by change of impedance of the sensor part due to change of gap with the rotating body. Also, the excitation current of the electromagnetic is controlled by using a displacement signal obtained by demodulating this amplitude modulated signal (for example, see Japanese Unexamined Patent Publication No. H6-313426).
In the case of the displacement sensor on the axial axis, because it is not opposed type, the offset value of the sensor signal changes according to a change of the floating capacitance of the sensor part or the signal line. In particular, when the cable connecting the magnetic bearing device and the controller is changed, the offset value tends to change greatly according to the change of floating capacitance of the cable. Accordingly, it was necessary to perform offset adjustment each time it is change. Therefore, in order to be able to eliminate the task of offset adjustment, the frequency of the carrier wave tends to be set low.
However, in case that the frequency of the carrier wave was set low, when removing the base wave of the carrier wave and the higher harmonics from the amplitude modulated wave in order to filter the sensor signal, the frequency of the carrier wave becomes relatively close to the required band of the sensor signal. Therefore, in order that the magnetic levitation controllability not be lowered, it was necessary to avoid phase delay of the required band to the greatest extent possible, and with regard to filtering, it was necessary to use a large number of notch filters instead of a low-pass filter. As a result, there was a drawback that it led to increase of circuit size.
The present invention has been made to avoid drawbacks of the conventional apparatus.
Further objects and advantages of the invention will be apparent from the following description of the invention.